Vascular remodeling in pulmonary arterial hypertension (PHTN) is a heterogeneous disorder which varies depending upon the type of injury, suggesting that smooth muscle cells within a vessel wall respond in distinct ways to different stimuli. Our preliminary data suggests that the medial thickening and vascular remodeling associated with PHTN does not result from a uniform proliferation of all smooth muscle cells within the pulmonary vessel, but from selective activation of subsets of pulmonary artery smooth muscle cells (PA SMC). Since the cause of PHTN can vary between individuals, understanding how subtypes of PA SMC respond to different types of injury has potential therapeutic implication. We hypothesize that subsets of PA SMC, identified by their ability to escape G0/G1 arrest following vascular injury, are responsible for the vessel remodeling in PHTN. Using cell culture, organ culture of extra-lobar and resistance pulmonary vessels, and in vivo experiments from both wild type and cyclin-dependent kinase inhibitor- (p21Cip1/Waf1 and p27Kip1) deficient animals we will examine: 1) how G1 cell cycle proteins are regulated in 'proliferative' compared to 'non-proliferative' PA SMC following stimulation, 2) the role of 3 intracellular signaling pathways which target G1 cell cycle proteins (RhoA/Rho kinase, P(3) kinase/AKT, and CREB) on regulating PA SMC proliferation within these subtypes, and 3) the effectiveness of three clinically relevant therapies (prostacyclin, endothelin blockers, and HMG CoA reductase inhibitors (statins)) in controlling proliferation in these subset(s) of PA SMC following injury. We anticipate that at the conclusion of this proposal we will have identified how subsets of PA SMC are selectively induced to proliferate in response to vascular injury. This information may lead to more targeted therapy for the treatment of PHTN.